LAST week, I met an artist friend who works out of South Block, a studio complex tucked behind Glasgow's busy pedestrian section of Argyle Street. South Block is a busy creative hub on Osborne Street, filled with artists and vibrant cultural businesses. During my visit, the fire alarm went off and everyone in the building had to evacuate the building. As we stood outside, I asked my friend: "Isn't one of the Billy Connolly murals on this street?" He looked at me incredulously. "You missed it?"

I looked up the street in the direction of St Enoch Centre. Sure enough, there it was on the side of a brick wall. A 50-foot copy of a portrait of national treasure, Sir William Connolly, at the grand old age of 75 as captured by his friend, artist and writer, John Byrne. Larger than life and hard to miss. Unless you are walking in the opposite direction with your back to it.

"It's not going down well in here," my friend said, rolling his eyes. "The people who work here are not happy that it just went up with no notice and now they have to live with it…"

Later, we wandered up to meet Byrne's Billy. There was a small crowd taking selfies. One well-spoken man offered to take a picture of me and my friend. There's a small private car-park in front of the mural and a hand-painted sign reads "No photos beyond this point".

The John Byrne version on Billy Connolly is one of three new murals of the Glasgow-born musician, comedian and actor which can now be seen on gable ends around Glasgow city centre. The others, by Jack Vettriano and Rachel Maclean, are a short walk away from the Byrne Billy. Vettriano's Connolly is on Dixon Street on the side of a pub behind the St Enoch Centre and Maclean's Billy is on a gable end just down from The Barras on the Gallowgate.

These three murals have joined a throng of murals dotted around Glasgow and are now officially part of Glasgow City Council’s City Centre Mural Trail project, an ever-expanding outdoor gallery which is a key part of the council's Glasgow City Centre Action Plan.

The Connolly murals emerged from a BBC Scotland documentary called Billy Connolly: Portrait of a Lifetime, which screened earlier this month. Made to mark Connolly's 75th birthday, BBC Scotland commissioned three portraits from Byrne, Singing Butler painter Vettriano, and Maclean, currently representing Scotland at prestigious contemporary art showcase, the Venice Biennale.

I enjoyed the programme very much. It presented a moving and quirky portrait of Connolly, who is suffering from Parkinson's Disease and cancer. Although increasingly frail, his ready wit and charm shone through. Interspersed with film footage from a stellar career, the main premise was to show how three very different visual artists portrayed this capricious creative spirit.

We witnessed Connolly's initial meetings with all three artists and then "the big reveal" as the Big Yin saw the portraits for the first time. Connolly declared his love for all three.

Byrne's portrait presented a mug-shot of the present, showing his old friend staring uncompromisingly back at him, with a sign reading 75BC-2017AD across his chest.

A star-struck Vettriano chose to paint a scene from Connolly's World Tour of Scotland series from 1994, featuring a windblown Billy on a storm-lashed coast near John O’Groats.

Maclean, who normally casts herself in her wild psychedelic and surreal digital films, styled Connolly as a latter-day Bonnie Prince Charlie figure in a specially created outfit, studded with motifs from his long career, including a mini bike parked in a bum (a reference to an infamous joke related during a Michael Parkinson television interview) and a sporran with an old nose sprouting hair.

The three original portraits are now on show at The People's Palace on Glasgow Green. I took a look at them the other day and for me, it's the Maclean digital print which stands out. By not taking the Big Yin at face value, she has mapped out his professional life and work through telling detail.

It's easy to criticise Vettriano and I always approach his work looking to be positive but I'm afraid this portrait is wooden and contrived. It reminded me of a painting you see in a school corridor by a fourth year pupil who shows potential. It doesn't even look like Connolly. As for the hands… well one of them is cut off and the other is wonky. Strangely, it works better on the gable end.

Byrne is a born draftsman and his portrait is tender if a wee bit lacking in his usual oomph. Maclean’s garish Connolly is a burst of surreal energy in an otherwise dreich street.

Writ large on gable ends, the portraits are taken to another, almost mawkish, level. In a city which is famous for its environmental art, producing more Turner Prize winners than you can shake a stick at, it seems bizarre that the best the city can do when it comes to its immediate built environment is throw up a bunch of murals. And I’m not just talking about the Connolly murals.

The big Billys are hard-to-miss and unsubtle. This might have been an apt description of The Big Yin in his sweary heyday, but these days we know there is so much more to him than meets the eye.

The public love Billy Connolly though. And from what I could see on my travels around the giant Billy murals and the originals from which they sprung, they love these murals too. The contemporary art world is not so enamoured. But that's another story and one which I suspect is to be continued…

Billy Connolly Murals Project. Find the portraits at People's Palace, Glasgow Green, and the murals at Osborne Street, Dixon Street & Gallowgate, Glasgow.